Export the EnvelopeVerifier type

[nmiyake]

Restore dsse.EnvelopVerifier as an external type as it was
in versions 0.1.0 and 0.2.0.

Fixes #11

[nmiyake]

@adityasaky would appreciate a review, merge, and release with this change if possible.

Without this change, we're unable to update our dependency of this module past version 0.2.0 because we have defined functions that use variables of type dsse.EnvelopeVerifier and it is no longer possible to do so because the type was made unexported in 0.3.0 (despite the fact that other exported functions still return values of this type).

[coveralls]

Pull Request Test Coverage Report for Build 1896392137

• 4 of 4 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 93.913%

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Totals
Change from base Build 1549769616:	0.0%
Covered Lines:	216
Relevant Lines:	230

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💛 - Coveralls

[adityasaky]

@nmiyake thanks for the ping, here's https://github.com/secure-systems-lab/go-securesystemslib/releases/tag/v0.3.1 for you :)

[nmiyake]

Thank you, much appreciated!