[FEATURE]: [Auth] Edgio REST API Centralized Authentication

Description

Having a centralized auth function that generates and set a global module variable with the access token allow a single-auth strategy for the SDK, allowing the user to use any other function on the SDK without needing to worry of authenticating a second time, or pass a context around to perform multiple operations withing Edgio's REST API.

Proposed Solution

As a developer, I want to be able to be able to authenticate to Edgio's REST API with my API Client's credentials and generate my access token to be able to use all packages available on 42dx/edgio-go-sdk.

Acceptance Criteria

[x] Edgio's REST API Auth function implementing and working
[x] Edgio's REST API Access token being generated and saved to a package's variable to be used by other functions
[x] Implement app.cache+app.cache.purge+app.waf+app.waf:edit+app.waf:read+app.accounts+app.config (all scopes) as default scope value
[x] Implement https://id.edgio.app/connect/token as default auth URL
[x] Unit test the implemented functions
[x] Document the usage

References

Edgio REST API Auth OfficialDocumentation

Code of Conduct

[X] I agree to follow this project's Code of Conduct

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42dx / edgio-go-sdk