[5.0 -> main] Fix base64 encoding - take 2

[heifner]

#1482 fixed base64 encoding by removing the unneeded extra = that variant added to base64 encoded data. However, it also updated the base64 library with a stricter one. This PR leaves the old base64 encoding/decoding library but does remove the extra = from the base64 encoded data so non-fc base64 decoders will not claim the data is invalid.

This change still breaks backward compatibility with 3.2 & 4.0 cleos/nodeos. PRs to 3.2 (#1889) and 4.0 will be created that will allow them to work with this new base64 encoding (they will no longer expect the invalid = character).

PR #1886 reverts #1482. This PR provided an alternative fix for #1461.

Included in this PR is an optimization to the existing base64_decode to avoid a copy for our use-cases by returning a vector<char> instead of a string.
Included in this PR is a change to the nodeos_run_test.py to keep the keosd running with passed --leaving-running which is useful for local testing. This was used to manually test using 3.2 cleos with 5.0.

Merges release/5.0 into main including #1888

Resolves #1461

[greg7mdp]

Again, not a required change, just a suggestion for here and the few next instances of the same pattern.

Suggested change

	std::string b64_str(b64.begin(), b64.end());
	BOOST_CHECK_EQUAL(expected_output, b64_str);
	BOOST_CHECK_EQUAL(std::string_view(expected_output), std::string_view(b64.begin(), b64.end()));