Custom gas price

[jimni1222]

Proposed changes

This PR introduces fixes for setting custom gas unit price.

Added another constructor for DefaultGasProvider. The added constructor takes caver as a parameter and sets gasPrice via getGasPrice.

In addition, the create function that takes custom gas price as a parameter has been added to the classes implementing each TransactionTransformer.

In SmartContract class, when creating TransactionTransformer, change gasProvider of gasProvider as parameter.

Types of changes

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• Bugfix
• New feature or enhancement
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Related issues

close #76

Further comments

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