Disable onChain trigger by default

[khanhtc1202]

What this PR does / why we need it:

To avoid unexpected triggering deployments in chain, should make this onChain trigger as disabled by default.

Which issue(s) this PR fixes:

Fixes #

Does this PR introduce a user-facing change?:

NONE

[pipecd-bot]

Code coverage for golang is 31.53%. This pull request does not change code coverage.

[nghialv]

👍
Btw, In the future, we can allow specifying more policies in that OnChain field, such as the application owner can use labels to specify a whitelist of which can trigger the application.

[nghialv]

/lgtm

[nakabonne]

we can allow specifying more policies in that OnChain field

That's the case

[nakabonne]

/approve

[pipecd-bot]

This pull request is APPROVED by nakabonne.

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