Enable Dynatrace 6.3 Agent and public repo

[akirasoft1]

Added public repository containing Dynatrace Agent 6.3 and incremented agent version to 6.3

Previously Dynatrace support was baked into the buildpack but commented out due to lack of Dynatrace provided agent download location. We have addressed this.

We have a valid/signed CLA through the Dynatrace organization.

[cfdreddbot]

Hey akirasoft!

Thanks for submitting this pull request!

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[akirasoft1]

Publicized my org membership!