[EAGLE-1068] Upgrade Siddhi dependency to v5.1.4

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## Purpose
Purpose of this pull request is to upgrade the [Siddhi](https://mvnrepository.com/artifact/io.siddhi) dependency version to `v5.1.4`. With this, it'll also;
- Fix `alert-core` and tests to support new Siddhi APIs.
- Fix existing custom Siddhi extensions to support new Siddhi APIs.
- Improve extension loading (use annotation support instead of `siddhiext`).
- Add the following dependencies (since they are now separated from Siddhi core);
```xml
<dependency>
    <groupId>io.siddhi.extension.execution.regex</groupId>
    <artifactId>siddhi-execution-regex</artifactId>
    <version>5.0.5</version>
</dependency>

<dependency>
    <groupId>io.siddhi.extension.execution.string</groupId>
    <artifactId>siddhi-execution-string</artifactId>
    <version>5.0.5</version>
</dependency>
```

## Remarks
- Fixes https://issues.apache.org/jira/browse/EAGLE-1068
- https://issues.apache.org/jira/browse/EAGLE-1065

---
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---

Author: Grainier <grainier@wso2.com>

Closes #1003 from grainier/EAGLE-1068.
44 files changed