PR contains:
- fix for claiming & deleting prebuilds with immutable params
- unit test for claiming scenario
- unit test for deletion scenario

The parameter resolver was failing when deleting/claiming prebuilds
because a value for a previously-used parameter was provided to the
resolver, but since the value was unchanged (it's coming from the
preset) it failed in the resolver. The resolver was missing a check to
see if the old value != new value; if the values match then there's no
mutation of an immutable parameter.

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Signed-off-by: Danny Kopping <dannykopping@gmail.com>

Adds @spikecurtis  and @johnstcn as CODEOWNERS of the provisioner protocol files. These need to be versioned, so we need some human review over changes.

Fixes coder/internal#604

Fixes a data race in `agentscripts.Runner` where a concurrent `Execute()` call races with `Init()`. We hit this race during shut down, which is not synchronized against starting up.

In this PR I've chosen to add synchronization to the `Runner` rather than try to synchronize the calls in the agent. When we close down the agent, it's OK to just throw an error if we were never initialized with a startup script---we don't want to wait for it since that requires an active connection to the control plane.

…sdk.Schema (#17632)