CLI for Remote Config Experiments

[Namyalg]

Description

This PR adds the implementation of the CLI utility for Remote Config Experiments. The operations exposed are:

1. GetExperiment
2. ListExperiments
3. DeleteExperiment

Scenarios Tested

1. Unit tests have been written
2. Manual E2E tests have been performed for the CLI commands.
3. The CLI wraps raw API errors into FirebaseError and returns it
4. Verify with the --json flag for Get, List and Delete experiments

GetExperiment and DeleteExperiment share similar test cases:

Expectation: The user should pass the experimentID that is required to be fetched / deleted.

Test cases:

Test Case	Description	Expectation
i	Passing a valid experimentId	Return the specified experiment.
ii	Not passing an experimentId	Returns an error about resource being invalid.
iii	Passing an incorrect / invalid ID	Returns an error about resource being invalid.

ListExperiment

Expectation: The user can optionally pass a filter, page_size and page_token.

Test Case	Description	Expectation
i	Passing none of the filter, page_size and page_token.	Return all experiments
ii	Passing a valid filter	Return the filtered experiment(s)
iii	Passing only a valid page size	Return page_size number of experiments
iv	Passing only a page token	Return 10 experiments after the page_token or remaining experiments (minimum of both)
v	Passing page_token and page_size	Return page_size number of experiments starting at page_token
vi	Pass a combination of page_token, page_size and filter	Return the filtered experiment(s)
vii	Pass an incorrect page_token	Return an error stating token is invalid
viii	Pass an incorrect / invalid filter	Return an empty object or no experiments

Sample Commands

1. GetExperiment

firebase remoteconfig:experiments:get <experiment ID>

2. ListExperiment

firebase remoteconfig:experiement:list --filter <optional filter> --page_size <optional page size> --page token <optional page token>

3. DeleteExperiment

firebase remoteconfig:experiments:delete <experiment ID>

[ashish-kothari]

LGTM overall. Let's also update https://github.com/firebase/firebase-tools/blob/master/README.md#remote-config-commands?

[joehan]

Please add a changelog entry - otherwise, this LGTM