Serialize/deserialize vfr key

[Dengjianping]

Description

Fix parsing vfr key in chain spec while start nodes by zombienet

Error: Service(Other("Error parsing spec file: missing field `vrf` at line 143 column 13"))
2023-07-22 23:26:52 Building chain spec    

Closes #1233

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• Connected to an issue with discussion and accepted design using zenhub "Connect issue" button below
• Added one label out of the L- group to this PR
• Added one or more labels from the A- and C- groups to this PR
• Explicitly labelled A-calamari and/or A-manta if your changes are meant for/impact either of these (CI depends on it)
• Re-reviewed Files changed in the Github PR explorer.

Situational Notes:

• If adding functionality, write unit tests!
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[github-actions]

✅ Congestion test: 1-day congestion cost (manta-runtime) is above the target daily congestion cost

[github-actions]

⚠️ Congestion test: 1-day congestion cost (calamari-runtime) is NOT above the target daily congestion cost