DRAFT - Bitget: Creating exchange implementation

[cranktakular]

PR Description

Implementing Bitget's V2 API, specifically the Common, Spot, Future, Margin, Earn, and Inst Loan sections.

In addition, does some smaller fixes across the codebase:

• Exports more errors.
• Lets types.Number support "null" values, interpreting them as 0.
• Removes GetPositionSummary, as there's already GetFuturesPositionSummary, which fulfills the same functionality.

Type of change

Please delete options that are not relevant and add an x in [] as item is complete.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How has this been tested

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration and also consider improving test coverage whilst working on a certain feature or package.

• go test ./... -race
• golangci-lint run
• Test X

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation and regenerated documentation via the documentation tool
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally and on Github Actions with my changes

Still needs two minor errors (GetOrderExecutionLimits intermittently failing; WsHandleData's error checks only function properly with Sonic) fixed. Also needs README reworded, and documentation regenerated. But it's largely in a position where it can be thoroughly tested and nitted.