feat: add `ExternalClientApplication` metadata support #1048

Job	Run time
megalinter	1m 28s
commit-lint	1m 15s
npm-lint	1m 13s
pull-request-lint	2s
build / source	2m 16s
publish-dev	1m 14s
e2e-check / e2e-test (macos-latest, 18, sfdx-cli)	1m 35s
e2e-check / e2e-test (macos-latest, 18, @salesforce/cli)	1m 10s
e2e-check / e2e-test (macos-latest, 20, sfdx-cli)	1m 16s
e2e-check / e2e-test (macos-latest, 20, @salesforce/cli)	1m 10s
e2e-check / e2e-test (windows-latest, 18, sfdx-cli)	5m 19s
e2e-check / e2e-test (windows-latest, 18, @salesforce/cli)	5m 24s
e2e-check / e2e-test (windows-latest, 20, sfdx-cli)	5m 56s
e2e-check / e2e-test (windows-latest, 20, @salesforce/cli)	6m 26s
e2e-check / e2e-test (ubuntu-latest, 18, sfdx-cli)	56s
e2e-check / e2e-test (ubuntu-latest, 18, @salesforce/cli)	40s
e2e-check / e2e-test (ubuntu-latest, 20, sfdx-cli)	58s
e2e-check / e2e-test (ubuntu-latest, 20, @salesforce/cli)	45s
	39m 3s

Provide feedback