Allow for configurable keystore / truststore type in ssl options

We've introduced two new configuration properties (spring.cloud.vault.ssl.key-store-type=…/spring.cloud.vault.ssl.trust-store-type=…) to configure the keystore type including PEM support.

Resolves gh-387.

docs/src/main/asciidoc/authentication.adoc

@@ -411,6 +411,7 @@ spring.cloud.vault:
     ssl:
         key-store: classpath:keystore.jks
         key-store-password: changeit
+        key-store-type: JKS
         cert-auth-path: cert
 ----
 ====

docs/src/main/asciidoc/new-features.adoc

@@ -7,3 +7,4 @@ This section briefly covers items that are new and noteworthy in the latest rele
 
 * Migration of `PropertySource` initialization from Spring Cloud's Bootstrap Context to Spring Boot's <<vault.configdata,ConfigData API>>.
 * Support for the <<vault.config.backends.couchbase>> backend.
+* Configuration of keystore/truststore types through `spring.cloud.vault.ssl.key-store-type=…`/`spring.cloud.vault.ssl.trust-store-type=…` including PEM support.

docs/src/main/asciidoc/other-topics.adoc

@@ -70,12 +70,14 @@ spring.cloud.vault:
     ssl:
         trust-store: classpath:keystore.jks
         trust-store-password: changeit
+        trust-store-type: JKS
 ----
 ====
 
 * `trust-store` sets the resource for the trust-store.
 SSL-secured Vault communication will validate the Vault SSL certificate with the specified trust-store.
 * `trust-store-password` sets the trust-store password
+* `trust-store-type` sets the trust-store type. Supported values are all supported `KeyStore` types including `PEM`.
 
 Please note that configuring `spring.cloud.vault.ssl.*` can be only applied when either Apache Http Components or the OkHttp client is on your class-path.