feat: make key prefix configurable

[jaimem88]

This PR will allow the Limiter to be optionally configured with a key prefix. This makes the limiter more flexible as not all applications need to be prefixed with rate:.

Using the Options pattern ensures we do not break backwards compatibility and will still use the default value.

Users of this package can chose to:

• Not use WithKeyPrefix when initializing the limiter to use the default value of rate:
• Set WithKeyPrefix("") to remove all prefixes from the used keys.
• Set whatever prefix they want.

I extended the TestAllow to use different key prefixes. I don't think it's necessary to extend the other tests as this already shows things work as expected.

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For extra context, we are looking at using this package for https://gitlab.com/gitlab-org/container-registry, however this is a current limitation we have given our guidelines.

[jaimem88]

@vmihailenco would you consider this change?

[jaimem88]

cc @joaodrp