feat(http_util): Implement multipart/mixed support

[Xuanwo]

This PR will add multipart/mixed support which makes it easier for developers to build requests with multipart/mixed content type.

[Xuanwo]

cc @ClSlaid, please take a look.

[suyanhanx]

😋It will be great if we could have a test for multi parts.(can be done in another PR.)
And we could also add a capability for this.

[Xuanwo]

yumIt will be great if we could have a test for multi parts.(can be done in another PR.)

I don't understand. Both MixedPart and FormDataPart are already tested.

[Xuanwo]

And we could also add a capability for this.

No. It's not a service capability.

[suyanhanx]

And we could also add a capability for this.

No. It's not a service capability.

😨I mean the functions which use them(the two part).

[Xuanwo]

I mean the functions which use them(the two part).

Maybe you mean this? 36c7cc0

[ClSlaid]

LGTM, good to see a fancy multipart implement!

[suyanhanx]

I mean the functions which use them(the two part).

Maybe you mean this? 36c7cc0

Oh sorry. I saw the test. I wasn't at the computer just now.
Yep, that's the capability. But we can check it with others together later.

[suyanhanx]

Great work! Thanks!