INT-4095: Support Limiting (S)FTP Files Fetched

JIRA: https://jira.spring.io/browse/INT-4095

Limit the number of remote files fetched on each poll (when it is necessary to fetch files).

Polishing - PR Comments

Polishing - Decouple MaxFetchSize from Poller

Polishing - PR Comments

Schemas and Docs

More Polishing

* Polishing according PR comments

spring-integration-ftp/src/main/resources/org/springframework/integration/ftp/config/spring-integration-ftp-5.0.xsd

@@ -164,6 +164,7 @@
 						</xsd:annotation>
 					</xsd:attribute>
 					<xsd:attributeGroup ref="tempSuffixGroup" />
+					<xsd:attributeGroup ref="integration:maxFetchGroup" />
 				</xsd:extension>
 			</xsd:complexContent>
 		</xsd:complexType>