feat(java): Implement user headers serialization / deserialization in java sdk

[slbotbm]

closes #2455

[slbotbm]

Since HeaderValue has String value in its signature, the current implementation converts all the values to string, ignoring the HeaderKind, which can indicate a non-string value.

[mmodzelewski]

Thanks, @slbotbm. I left some minor comments. Otherwise, the PR looks great.

[slbotbm]

@mmodzelewski thank you for the review! I have addressed all of your comments. After removing the optional mapping from userHeaders, some http integration tests were failing due to jackson desrializing empty userHeaders as null. To address this, I added the annotation @JsonSetter(nulls = Nulls.AS_EMPTY) so that Jackson would return an empty map.

[mmodzelewski]

@slbotbm thanks, added a few comments regarding the new code.

[mmodzelewski]

@slbotbm all looks great, thanks again for the contribution! :)