Add --local-config-method, merge local app components with remote ones by default

[skarekrow]

Fixes #300

This should obviate the original behavior of merging the local apps in totality and instead patch the apps specifed with the component overrides.

Special thanks to @rsnyman and @bsquizza for their help with this.

[bsquizz]

While I was at it I also renamed the 'local' source mode to 'file' because 'local' is a misleading name.

The local config is always parsed, even in 'appsre' mode. Really, what 'local' mode meant was that bonfire would fetch its apps config using the file defined at the appsFile in config.yaml instead of using app-sre. That mode is not currently used in the platform tooling so I do not think renaming it will have large impact, and that mode may prove useful again in the future so I kept the ability to define an appsFile in the code.

Whether your source of truth is app-sre or the 'apps file', the --local-config-method will operate the same way when it comes to merging your locally defined apps with the remotely defined ones.

[bsquizz]

When we release this, we should tag this as v5 since this is technically making a "breaking change" -- although I suspect it will impact very few people... and for the people it does impact, they just need a heads up about the new CLI flag if they desire to return to the old config merging behavior.

[Victoremepunto]

@skarekrow @bsquizz Can we add some E2E tests to keep this behavior away of future possible regressions ? they'll also help understand the behavior, I believe looking at a simplified use case with expectations is clearer than anything.

Thanks!

[bsquizz]

Latest commit keeps this all backward-compatible:

1. default merge method remains 'override' as it was before
2. the source named 'local' is kept around for now and there's a warning indicating that it is deprecated and has been renamed to 'file'

[bsquizz]

We're going to submit an ADR to get buy-in from leads on changing the default merge method

[bsquizz]

Latest commit keeps this all backward-compatible:

1. default merge method remains 'override' as it was before
2. the source named 'local' is kept around for now and there's a warning indicating that it is deprecated and has been renamed to 'file'

Commit 10b78b7 reverted since we'll get approval for the change in default behavior via ADR process.