Support Standalone Activity client in temporal-testing

[444am]

What was changed

Added ActivityClient support to temporal-testing, and updated README.
ℹ️ I understand that with this change, Standalone Activity would be still yet properly supported in local Test Server. However as the changes to that part would be mostly in temporal-test-server, I would like to keep this PR focusing on TestWorkflowEnvironment contract enrichment.

Why?

Standalone Activity tests had to manually construct an ActivityClient from service stubs and options. This PR makes standalone activity testing use the same test-environment API style as Workflow Client testing.

How was this tested:

See the new UT file

[maciejdudko]

Hi @444am, thank you for your contribution! The code looks great, no corrections needed except for a small change in README.md and removing TimeLockingActivityClientInterceptor class (see comments). In the interest of time, I'll apply these changes myself.

[maciejdudko]

Soon, every up-to-date Temporal server will have Standalone Activities supported and enabled by default; the last part will become dated very quickly.

Suggested change

	`TestWorkflowEnvironment`, `TestWorkflowRule`, and `TestWorkflowExtension` provide clients connected
	to the test service. Use `getWorkflowClient()` for Workflows and `getActivityClient()` for
	Standalone Activities when the target Temporal service supports Standalone Activity APIs.
	`TestWorkflowEnvironment`, `TestWorkflowRule`, and `TestWorkflowExtension` provide clients connected
	to the test service. Use `getWorkflowClient()` for Workflows and `getActivityClient()` for
	Standalone Activities.

[maciejdudko]

Time-skipping API is currently being reworked, and the test server doesn't support Standalone Activities yet. It means this interceptor is unusable currently and will likely remain unusable in its current form. I'd rather not include it in this PR, and have it added separately at a later date when the new time-skipping API are finalized.

TL;DR: remove this class.