Add secp256k1 & brainpool EC curves (rebased)

[sake]

This MR is based on the work of #60.

It rebases the work against the current main branch, solving a few minor merge conflicts.
It furthermore addresses the review comments regarding the println statements in EcdsaTest.kt.

[whyoleg]

Thanks!
I've started the CI checks and will merge after they finished.

[sake]

Regarding the OOM error in the Android tests, I looked at the code again and I can not see any place which could introduce a memory leak.
Did you have any problems running these tests in the CI?

[whyoleg]

I beleive there are no memory leaks there, but probably the issue with low memory or slow GC on android emulator.
In this PR we increased the amount of curves under testing and so more objects were generated.

Later today, I will merge some of my other changes, which will drop running tests for BouncyCastle on the Android emulator— it's unnecessary, as there is no difference in behavior between Android and JVM for it, and so there is no reason to test it.
After that, there should be no OOM.

[whyoleg]

@sake feel free to rebase, the relevant changes are merged to main