Add concurrency to host schema

[motm32]

This PR adds a concurrency property to the host schema and also adds tests.
This is needed in order to enable dynamic concurrency.

[GerryFerdinandus]

@motm32
Can you add "additionalProperties": false

According to the documentation
Dynamic concurrency is disabled by default.
But you set the default to 'true'

The default for snapshotPersistenceEnabled is correct.
When SnapshotPersistenceEnabled is true, which is the default,

[motm32]

@motm32 Can you add "additionalProperties": false

Will do!

According to the documentation Dynamic concurrency is disabled by default. But you set the default to 'true'

Since concurrency is not automatically enabled, if users are adding this setting to their host.json they should by default want it set to true. If they didn't want to enable dynamic concurrency this setting wouldn't be added to the host.json.

Here is a link to the vscode-azurefunctions issue for more info.

[GerryFerdinandus]

Thanks