Fix value of EntityType AGENT

[wiegandm]

The value of EntityType.AGENT in the various GMP protocols was note, but should be agent.

Checklist:

• Tests
• CHANGELOG Entry
• Documentation N/A

[codecov]

Codecov Report

Merging #190 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #190   +/-   ##
=======================================
  Coverage   91.38%   91.38%           
=======================================
  Files          12       12           
  Lines        1022     1022           
=======================================
  Hits          934      934           
  Misses         88       88

Impacted Files	Coverage Δ
gvm/protocols/gmpv8/types.py	100% <100%> (ø)	⬆️
gvm/protocols/gmpv7/types.py	100% <100%> (ø)	⬆️
gvm/protocols/gmpv9/types.py	94.49% <100%> (ø)	⬆️

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[bjoernricks]

Thanks!