Label selector support (#1539)

* Import labels module from linkerd

straight copy + factor out 2 std imports

Signed-off-by: clux <[email protected]>

* some initial interface thoughts

Signed-off-by: clux <[email protected]>

* Implement Selector to match on resource labels

Signed-off-by: Danil-Grigorev <[email protected]>

* Implement SelectorExt and a Matcher trait

- Add SelectorExt to the prelude
- Remove Map in favor of more explicit BTreeMap<String, String>
- Add exmples of usage

Signed-off-by: Danil-Grigorev <[email protected]>

* Use TryFrom for LabelSelector conversion, drop Expression::Invalid

Signed-off-by: Danil-Grigorev <[email protected]>

* Sealing the SelectorExt trait

- fmt fixes

Signed-off-by: Danil-Grigorev <[email protected]>

* Rename Matcher to SelectorExt and seal

- Implemented for Selector and Expression for now

Signed-off-by: Danil-Grigorev <[email protected]>

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Signed-off-by: clux <[email protected]>
Signed-off-by: Danil-Grigorev <[email protected]>
Co-authored-by: clux <[email protected]>