[group key addrs 2/?]: internal/ecies: add encrypt/decrypt with ECIES

[guggero]

Implements the ECIES encryption scheme as mentioned here: #874 (comment)

This commit creates a simple encryption and decryption function that uses AES256-GCM for tne symmetric encryption and HKDF with SHA256 for the key derivation.
The shared key generation is not part of these functions, because we'll need to use lnd RPCs in some cases to be able to derive it.

[coveralls]

Pull Request Test Coverage Report for Build 14855573634

Details

• 59 of 82 (71.95%) changed or added relevant lines in 1 file are covered.
• 15 unchanged lines in 5 files lost coverage.
• Overall coverage increased (+0.1%) to 37.071%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
internal/ecies/ecies.go	59	82	71.95%

Files with Coverage Reduction	New Missed Lines	%
address/address.go	2	69.55%
asset/group_key.go	2	57.89%
tapgarden/planter.go	2	60.85%
commitment/tap.go	4	71.82%
tapchannel/aux_leaf_signer.go	5	43.08%

Totals
Change from base Build 14790497726:	0.1%
Covered Lines:	26580
Relevant Lines:	71701

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💛 - Coveralls