Added all test cases and fixed bugs for Java (HttpClient) codegen

[royshruti]

PR Description

This PR fixes multiple bugs and adds additional support in the CodeGen for Java(HTTPClient) and also tests the generated code.

Bugs fixed/Support added:

• fixes the handling of query parameters as earlier the query parameters weren't parsed correctly.
• fixes the handling of headers
• adds a class for handling multipart/form-data exclusively
• effectively handles multipart/form-data(now can send both text and files ).
• fixes the HTTP request as PATCH and DELETE methods are not dedicated methods for HTTPClient.

Related Issues

• Related Issue #137
• Closes #137

Checklist

• [x] I have gone through the contributing guide
• [x] I have updated my branch and synced it with project main branch before making this PR
• [x] I have run the tests (flutter test) and all tests are passing

Added/updated tests?

We encourage you to add relevant test cases.

• [x] Yes

Added/updated tests?

• [x] Yes

Instructions on running the generated code on local computer

1. As HttpClient is a part of core java, we need to install JDK and JRE on our local system. You can install JDK binaries from Oracle.
2. We can compile the code using javac JavaHttpClientExample.java where JavaHttpClientExample.java is the file which contains generated code. Then we can run the generated class file using java JavaHttpClientExample.

[animator]

Provide Instructions to run the generated code in local computer.

[royshruti]

@animator I have added the instructions to run the generated code in local computer.

[animator]

@royshruti Please find below my feedback:

• The generated code has unused imports. Please import only what is being used.
• POST 1 is incorrect. Take a look at other codegen tests. You are not attaching relevant content-type header.
• Multipart code is very large. You can use Methanol to handle multipart - https://mizosoft.github.io/methanol/

[animator]

Closing this PR. You can open a new PR after making all the suggested changes.