Implement Provider interface for modular and non-modular apps.

[msgilligan]

This makes it possible to build applications and libraries with a sole compile-time dependency on secp256k1-api. At runtime either the native (aka foreign or ffm) or Bouncy Castle implementation (when complete) can be "used".

• Define the Secp256k1Provider interface and two implementations with no-arg constructors.
• Add the get() and getByName() methods to the Secp256k1 interface
• Update module-info files with provides and uses statements.
• Add resource files for service provider location in non-modular apps.
• Remove direct dependencies on implementations from the examples
• Run the Java example application as a modular application
• Switch the Kotlin example default app to Ecdsa.java

[msgilligan]

@schildbach I just converted this to Draft, because I think the following PRs should be reviewed/merged first:

• PR #44
• PR #45

Once those are merged this one can be rebased.

[msgilligan]

PR #47 breaks out one minor change from this PR that can be merged separately.

Update: So does PR #48

[schildbach]

I'll review this once the others are merged and this one rebased.

[msgilligan]

@schildbach Ready-for-review!