Introduce DefaultMultipartMessageReader

This commit introduces the DefaultMultipartMessageReader, a fully
reactive multipart parser that does have any third party dependencies.

Closes gh-21659

spring-web/src/test/resources/org/springframework/http/codec/multipart/.gitattributes

@@ -0,0 +1 @@
+*.multipart     -text

spring-web/src/test/resources/org/springframework/http/codec/multipart/chrome.multipart

@@ -0,0 +1,27 @@
+------WebKitFormBoundaryEveBLvRT65n21fwU
+Content-Disposition: form-data; name="text1"
+
+a
+------WebKitFormBoundaryEveBLvRT65n21fwU
+Content-Disposition: form-data; name="text2"
+
+b
+------WebKitFormBoundaryEveBLvRT65n21fwU
+Content-Disposition: form-data; name="file1"; filename="a.txt"
+Content-Type: text/plain
+
+Lorem ipsum dolor sit amet, consectetur adipiscing elit. Integer iaculis metus id vestibulum nullam.
+
+------WebKitFormBoundaryEveBLvRT65n21fwU
+Content-Disposition: form-data; name="file2"; filename="a.txt"
+Content-Type: text/plain
+
+Lorem ipsum dolor sit amet, consectetur adipiscing elit. Integer iaculis metus id vestibulum nullam.
+
+------WebKitFormBoundaryEveBLvRT65n21fwU
+Content-Disposition: form-data; name="file2"; filename="b.txt"
+Content-Type: text/plain
+
+.mallun mulubitsev di sutem silucai regetnI .tile gnicsipida rutetcesnoc ,tema tis rolod muspi meroL
+
+------WebKitFormBoundaryEveBLvRT65n21fwU--

spring-web/src/test/resources/org/springframework/http/codec/multipart/firefox.multipart

@@ -0,0 +1,27 @@
+-----------------------------18399284482060392383840973206
+Content-Disposition: form-data; name="text1"
+
+a
+-----------------------------18399284482060392383840973206
+Content-Disposition: form-data; name="text2"
+
+b
+-----------------------------18399284482060392383840973206
+Content-Disposition: form-data; name="file1"; filename="a.txt"
+Content-Type: text/plain
+
+Lorem ipsum dolor sit amet, consectetur adipiscing elit. Integer iaculis metus id vestibulum nullam.
+
+-----------------------------18399284482060392383840973206
+Content-Disposition: form-data; name="file2"; filename="a.txt"
+Content-Type: text/plain
+
+Lorem ipsum dolor sit amet, consectetur adipiscing elit. Integer iaculis metus id vestibulum nullam.
+
+-----------------------------18399284482060392383840973206
+Content-Disposition: form-data; name="file2"; filename="b.txt"
+Content-Type: text/plain
+
+.mallun mulubitsev di sutem silucai regetnI .tile gnicsipida rutetcesnoc ,tema tis rolod muspi meroL
+
+-----------------------------18399284482060392383840973206--

spring-web/src/test/resources/org/springframework/http/codec/multipart/part-no-end-boundary.multipart

@@ -0,0 +1,5 @@
+--boundary
+Header: Value
+
+a
+--boundary

spring-web/src/test/resources/org/springframework/http/codec/multipart/part-no-header.multipart

@@ -0,0 +1,4 @@
+--boundary
+
+a
+--boundary--

spring-web/src/test/resources/org/springframework/http/codec/multipart/safari.multipart

@@ -0,0 +1,27 @@
+------WebKitFormBoundaryG8fJ50opQOML0oGD
+Content-Disposition: form-data; name="text1"
+
+a
+------WebKitFormBoundaryG8fJ50opQOML0oGD
+Content-Disposition: form-data; name="text2"
+
+b
+------WebKitFormBoundaryG8fJ50opQOML0oGD
+Content-Disposition: form-data; name="file1"; filename="a.txt"
+Content-Type: text/plain
+
+Lorem ipsum dolor sit amet, consectetur adipiscing elit. Integer iaculis metus id vestibulum nullam.
+
+------WebKitFormBoundaryG8fJ50opQOML0oGD
+Content-Disposition: form-data; name="file2"; filename="a.txt"
+Content-Type: text/plain
+
+Lorem ipsum dolor sit amet, consectetur adipiscing elit. Integer iaculis metus id vestibulum nullam.
+
+------WebKitFormBoundaryG8fJ50opQOML0oGD
+Content-Disposition: form-data; name="file2"; filename="b.txt"
+Content-Type: text/plain
+
+.mallun mulubitsev di sutem silucai regetnI .tile gnicsipida rutetcesnoc ,tema tis rolod muspi meroL
+
+------WebKitFormBoundaryG8fJ50opQOML0oGD--