exchanges/gateio: Add websocket order batch [SPOT] support with context mocking

[shazbert]

PR Description

• Add wrapper interface function WebsocketSubmitBatchOrders
• Add gateio wrapper function WebsocketSubmitBatchOrders for spot only
• Adds context mock support for websocket and REST. This is just a basic POC implementation, in future we could take in an interface to our mock server but for now what you put in is what it is going to return. The websocket specifically will generate a mock connection when you get the connection for the request itself, so no need setup or connect, but will allow the full return functionality to be tested for an exchange wrapper. This can be run during live testing as well complimentary for both rest and WS and full coverage can be obtained.

Futures not done just to keep this as small as possible and I haven't done any trading batch wise to verify it yet.

Requires #1603

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
• Any dependent changes have been merged and published in downstream modules