Update SAS expiry DateTime creation

[vdiddlodka]

Created new method to produce properly formatted expiry DateTime strings. Azure did not accept standard PHP generated ISO 8601 datetime with DateTime::ATOM as suggested in PHP docs. Instead had to produce the datetime string in Zulu. Also updated documentation around implementation of generateBlobServiceSharedAccessSignatureToken method.

[ghost]

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[spaze]

@vdiddlodka Try the DateTime format used in #294 when using non-UTC timestamps.

[vdiddlodka]

I'm receiving an error when trying to sign CLA, "Error
There is no CLA to sign for Azure/azure-storage-php

({"message":"Bad credentials","documentation_url":"https://docs.github.com/rest"})"

[vdiddlodka]

@vdiddlodka Try the DateTime format used in #294 when using non-UTC timestamps.

Edit 'p' was introduced in PHP 8.0 but 'P' should be available in PHP 7.

I thought 'P' in format was introduced in PHP 8 which would increase the minimum PHP version requirements. I could be wrong on that however.

[spaze]

@vdiddlodka Try the DateTime format used in #294 when using non-UTC timestamps.

I thought 'P' in format was introduced in PHP 8 which would increase the minimum PHP version requirements. I could be wrong on that however.

Lowercase p was indeed added in PHP 8 (see changelog) but I'm using uppercase P in the change. Uppercase P works since the DateTime class was added to PHP (at least since 5.2). This SDK targets PHP 5.6 and newer.

[vdiddlodka]

For reference, I found this link detailing the accepted datetime format by the API; https://docs.microsoft.com/en-us/rest/api/storageservices/formatting-datetime-values