Update NewAdminClientParams to support AccessToken (#122)

## Problem
While implementing admin API functionality in the CLI through the Go
SDK, I realized we need a way to provide an `AccessToken` directly
rather than strictly `ClientId` and `ClientSecret`. In cases where the
user has authenticated through the browser and OAuth, they already have
the token and may not have provided `ClientId` and `ClientSecret`.

Additionally, I noticed a few things with our generated Go documentation
that needed to be cleaned up. The interfaces for `ProjectClient`,
`OrganizationClient`, and `ApiKeyClient` are somewhat buried in our
generated documentation. Here's an example of `ProjectClient`:
https://pkg.go.dev/github.com/pinecone-io/go-pinecone/v4/pinecone#ProjectClient.

The concrete implementations of the interfaces that all the methods are
attached to need to be properly exported.

## Solution
- Update `NewAdminClientWithContext`, and `NewAdminClientParams` to
support passing `AccessToken` instead of `ClientId` and `ClientSecret`,
add new unit tests to cover `NewAdminClientParams` functionality.
- Update `ProjectClient`, `OrganizationClient`, and `ApiKeyClient`
interfaces and concrete implementations to export things properly to
correct documentation issues. Move detailed doc comments to the specific
methods rather than having them in the interfaces.
- Un-export IntegrationTests and AdminIntegrationTests.
- Add unit tests for `NewAdminClientWithContext`.

## Type of Change
- [ ] Bug fix (non-breaking change which fixes an issue)
- [X] 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
- [ ] Infrastructure change (CI configs, etc)
- [ ] Non-code change (docs, etc)
- [ ] None of the above: (explain here)

## Test Plan
CI - integration & unit tests

Author: austin-denoble