feat: add file resource provider for EG standalone mode

[shawnh2]

What type of PR is this?

What this PR does / why we need it:

This PR implement two component for file resource provider: Notifier and ResourceStore.

• Notifier monitors paths(files or dirs) on host via fsnotify, and send Write/Remove event to ResourceStore
• ResourceStore load and store resources according to the received event

Which issue(s) this PR fixes:

related #1393

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There're some TODOs in this PR, tracked by #3213 #3207

[zhaohuabing]

As discussed in today's meeting, I prefer to remove the EnvoyGatewayCustomProvider and add a flat EnvoyGatewayFileProvider(or a better name).

[arkodg]

As discussed in today's meeting, I prefer to remove the EnvoyGatewayCustomProvider and add a flat EnvoyGatewayFileProvider(or a better name).

Custom is needed to allow for different resource and infra provider

[zhaohuabing]

As discussed in today's meeting, I prefer to remove the EnvoyGatewayCustomProvider and add a flat EnvoyGatewayFileProvider(or a better name).

Custom is needed to allow for different resource and infra provider

Add it later when a use case comes out? The File/Host Provider is not a custom provider, it's a known one.

[arkodg]

As discussed in today's meeting, I prefer to remove the EnvoyGatewayCustomProvider and add a flat EnvoyGatewayFileProvider(or a better name).

Custom is needed to allow for different resource and infra provider

Add it later when a use case comes out? The File/Host Provider is not a custom provider, it's a known one.

use case exists today - file resource provider and host infra provider to implement standalone case, running EG in a docker container or bare metal directly w/o any API server

[Xunzhuo]

/retest

[Xunzhuo]

Do we plan on releasing this in v1.1.0 ?

cc @arkodg

[arkodg]

Do we plan on releasing this in v1.1.0 ?

cc @arkodg

yeah I'm a +1 for this, prioritizing other PRs right now, will get to this one soon

[shawnh2]

Some tests will be added after the first round of reviews.