Introduce new configuration option `entity_attributes` that defines a list of
dictionaries each of which represents an <Attribute> element. Each dicrionary has fields
for the NameFormat, the Name, the FriendName and a list of strings that are used to
create <AttributeValue> elements, each with the string as the text node.

    "entity_attributes": [
      {
        "name_format": "urn:oasis:names:tc:SAML:2.0:attrname-format:uri",
        "name": "urn:oasis:names:tc:SAML:profiles:subject-id:req",
        # "friendly_name" is not set
        "values": ["any"],
      },
    ]

Signed-off-by: Ivan Kanakarakis <ivan.kanak@gmail.com>

Signed-off-by: Ivan Kanakarakis <ivan.kanak@gmail.com>

The `name_id_format` configuration option is used to define

1. the value of the `<NameIDFormat>` metadata element
2. and the value of the `<NameIDPolicy>` `Format` attribute in an `AuthnRequest`

The configuration option to set what the value of `<NameIDFormat>` element is in the
metadata should be different from the configuration option to specify what should be
requested in an `AuthnRequest` through the `<NameIDPolicy Format="...">` attribute.

Introduce a new option (`name_id_policy_format`), or use the same name but scoped in a
specific section for metadata and AuthnRequest.

On the side of this, pysaml2 defaults to _transient_ as the `<NameIDPolicy
Format="...">` attribute value. To omit requesting a value for the `<NameIDPolicy
Format="">` attribute the value `"None"` (a string) must be set in the configuration.
This is unintuitive. It is better to be explicit and set transient to request a
transient NameID, than not setting a value and requesting transient by default. If no
value is set, no specific `<NameIDPolicy Format="...">` should be requested.

- Refactor the name_id_format usage
- Add name_id_policy_format configuration option
- Remove the "None" convention value

Signed-off-by: Ivan Kanakarakis <ivan.kanak@gmail.com>

Mention `additional_cert_files` and the fact that `cert_file` only accepts a single cert and not a chain

Document the default value for 'want_response_signed' so users don't
have to dig through the code to fine it.