Document 'base64:' prefix support

Closes gh-43809

Author: snicoll

spring-boot-project/spring-boot-docs/src/docs/antora/modules/reference/pages/features/external-config.adoc

@@ -1188,6 +1188,22 @@ Doing so gives a transparent upgrade path while supporting a much richer format.
 
 
 
+[[features.external-config.typesafe-configuration-properties.conversion.base64]]
+==== Converting Base64 Data
+
+Spring Boot supports resolving binary data that have been Base64 encoded.
+If you expose a `Resource` property, the base64 encoded text can be provided as the value with a `base64:` prefix, as shown in the following example:
+
+[configprops%novalidate,yaml]
+----
+my:
+  property: base64:SGVsbG8gV29ybGQ=
+----
+
+NOTE: The `Resource` property can also be used to provide the path to the resource, making it more versatile.
+
+
+
 [[features.external-config.typesafe-configuration-properties.validation]]
 === @ConfigurationProperties Validation
 

spring-boot-project/spring-boot-docs/src/docs/antora/modules/reference/pages/features/ssl.adoc

@@ -43,6 +43,12 @@ When used to secure a client-side connection, a `truststore` is typically config
                 password: "secret"
 ----
 
+[TIP]
+====
+Rather than the location to a file, the xref:features/external-config.adoc#features.external-config.typesafe-configuration-properties.conversion.base64[Base64 encoded content] of the file can be provided.
+If you chose this options, the value should start with `base64:`.
+====
+
 See javadoc:org.springframework.boot.autoconfigure.ssl.JksSslBundleProperties[] for the full set of supported properties.
 
 NOTE: If you're using environment variables to configure the bundle, the name of the bundle is xref:features/external-config.adoc#features.external-config.typesafe-configuration-properties.relaxed-binding.maps-from-environment-variables[always converted to lowercase].
@@ -84,6 +90,9 @@ When used to secure a client-side connection, a `truststore` is typically config
 
 [TIP]
 ====
+Rather than the location to a file, the xref:features/external-config.adoc#features.external-config.typesafe-configuration-properties.conversion.base64[Base64 encoded content] of the file can be provided.
+If you chose this options, the value should start with `base64:`.
+
 PEM content can be used directly for both the `certificate` and `private-key` properties.
 If the property values contain `BEGIN` and `END` markers then they will be treated as PEM content rather than a resource location.