span type/subtype: alignment step 1

[SylvainJuge]

• add generic app/internal to replace some internal spans & deprecate those.
• add to specification that changing span type/subtype is not considered a breaking change.
• deprecate some fields listed in Alignment of span type/subtype #502 before removal.

[apmmachine]

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• Start Time: 2021-12-02T08:23:47.061+0000

• Duration: 7 min 49 sec

• Commit: 01d9b55

[SylvainJuge]

Since this PR does not introduce any breaking change in agents, we can merge it as-is.
A follow-up PR will have to be created afterwards to remove the deprecated fields, this time probably having more impact on agents that do enforce this spec.