Rename `SignedState` to `SignableState`

## 🆕🐥 First Timers Only

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We know that creating a pull request (PR) is a major barrier for new contributors.
The goal of this issue and all other issues labeled by [**'good first issue'**](https://github.com/issues?q=is%3Aopen+is%3Aissue+org%3Ahashgraph+archived%3Afalse+label%3A%22good+first+issue%22+) is to help you make your first contribution to Hedera.

## 👾 Description of the issue

There is a class `SignedState.java`. This class is the top level object of the state and has a field where signatures are added to it as they are collected. It collects signatures in this class level field:

```
/**
* the signatures collected so far (including from self)
*/
private SigSet sigSet;
```

Once enough signatures are collected, the state is considered "signed". The name of this class is misleading because it implies that the state is always signed, but there is time before it collects enough signatures during which it is NOT signed. See in the `SignedState.java` constructor the `sigSet` is initialized with no signatures:

`sigSet = new SigSet();`

Signatures are added to the `sigSet` later, as they are collected, using this method:

```
/**
* Add a signature to the sigset. Does not validate the signature.
*
* @param nodeId the ID of the node that provided the signature
* @param signature the signature to add
*/
public void addSignature(@NonNull final NodeId nodeId, @NonNull final Signature signature)
```

### Steps to reproduce

Find the `SignedState` class. Observe the empty `sigSet` in the constructor.

### Proposed Solution:

Refactor the `SignedState.java` class to `SignableState.java`. This new name is more accurate and clear, indicating that it is an object that can collect signatures but does not imply how many have been collected.

The refactor should include all test classes with the term `SignedState` in the name.

Leave the `SignedStateInfo.java` class alone. There is [another ticket](https://github.com/hashgraph/hedera-services/issues/15304) for removing this unused interface.

## 📋 Step by step guide to do a contribution

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A more detailed general documentation of the GitHub PR workflow can be found [here](https://github.com/firstcontributions/first-contributions/blob/master/README.md).

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## 🤔 Additional Information

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