Issue: SPR-15497

Issue: SPR-15474

This comment extends the use of the charset property in
FormHttpMessageConverter to also include multipart headers with a
default of UTF-8.

We now also set the charset parameter of the "Content-Type" header to
indicate to the server side how to decode correctly.

Issue: SPR-15205

The MultipartHttpMessageWriter now directly encodes part header values
defaulting to UTF-8 and also specifies the charset in the
Content-Type header for the entire request.

This should work with something commonly used like Apache Commons
FileUpload which checks request.getCharacterEncoding() and uses it
for reading headers.

This commit adds a `toParts` method in `BodyExtractors`, returning a
BodyExtractor<Part>.

Issue: SPR-13779

* pr/1409:
  Simplify Netty dependency management by importing its bom

Closes gh-1409

Issue: SPR-14546

Issue: SPR-14546

This commit turns the Synchronoss NIO Multipart HttpMessageReader into
a reader of Flux<Part> and creates a separate reader that aggregates
the parts into a MultiValueMap<String, Part>.

Issue: SPR-14546

Issue: SPR-15485

Issue: SPR-14512

This commit properly closes the opened channels in the SynchronossPart,
and also makes sure that the entire contents is copied, not just the
first batch.

Includes a revision of the AutowireCandidateResolver SPI with default methods.

Issue: SPR-15268

Update `SpringFactoriesLoader` to cache property file loads. This helps
reduce the number of garbage objects created as each load uses an 8K
char buffer.

Issue: SPR-15509

This commit introduces a `writableChannel()` method to
`WritableResource`, defaulting to `Channels.newChannel`, but with
overrides for file-based resources.

Issue: SPR-15510

Issue: SPR-15499

Issue: SPR-15504

Issue: SPR-15506

Issue: SPR-15508

Issue: SPR-14546

Issue: SPR-15507

Issue: SPR-15503

Change the `StringHttpMessageConverter` to defer calling
Charset.availableCharsets() until absolutely necessary to help improve
startup times.

Issue: SPR-15502